[Ada] Use function and not procedure UI_Image in pretty-printing

[Pierre-Marie de Rodat]

Code cleanup; semantics is unaffected. The code for pretty-printing
integer, real and string literals now looks the same.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * pprint.adb (Expr_Name): Simplify with functional variant of
        UI_Image.
diff --git a/gcc/ada/pprint.adb b/gcc/ada/pprint.adb
--- a/gcc/ada/pprint.adb
+++ b/gcc/ada/pprint.adb
@@ -226,8 +226,7 @@ package body Pprint is
                end;
 
             when N_Integer_Literal =>
-               UI_Image (Intval (Expr));
-               return UI_Image_Buffer (1 .. UI_Image_Length);
+               return UI_Image (Intval (Expr));
 
             when N_Real_Literal =>
                return Real_Image (Realval (Expr));